1. Field of the Invention
The present invention relates to nonreciprocal circuit devices and communications devices used for high-frequency bands, particularly in submillimeter-wave bands.
2. Description of the Related Art
A known example of a nonreciprocal circuit device will be illustrated with reference to FIG. 10. FIG. 10 shows an exploded perspective view of a nonreciprocal circuit device, which is commonly referred to as a xe2x80x9clumped-constant nonreciprocal circuit devicexe2x80x9d.
As shown in FIG. 10, a nonreciprocal circuit device 110 comprises an upper yoke 111 and a lower yoke 112 for forming a closed magnetic circuit, three central conductors 121, 122, and 123, a ferrite member 120 having the central conductors thereon, a magnet 113 for applying a DC magnetic field to the ferrite member 120 having the central conductors, and a resin case 130. The three central conductors 121, 122, and 123 formed on the ferrite member 120 mutually intersect at an angle of 120xc2x0 via an insulation film (not shown). One of the ends of each central conductor is an input/output terminal. The other ends thereof are ground terminals and they are all disposed on the lower surface of the ferrite member 120. In the resin case 130, a hole 131 for receiving the ferrite member 120 having the central conductors 121, 122, and 123, recesses 132 and 136 for receiving capacitors 115 and a resistor 114, and input/output connection electrodes 133 for connecting to input/output terminals are formed. In addition, the input/output connection electrodes 133 are connected to respective terminal electrodes 135 on an outer surface of the resin case 130, and an electrode connected to an end of the resistor 114 and the back of the capacitor 115 is connected to another terminal electrode 135 on the outer surface of the resin case 130.
The input/output terminal P1xe2x80x2 of the central conductor 121 and the input/output terminal P2xe2x80x2 of the central conductor 122 are connected to respective input/output connection electrodes 133 formed in the resin case 130 and the top electrodes of two of the capacitors 115, respectively, whereas the input/output terminal P3xe2x80x2 of the central conductor 123 is connected to the top electrode of the third capacitor 115 and an electrode of the resistor 114.
FIG. 11 shows an equivalent circuit diagram of the nonreciprocal circuit device 110 having the above structure. The central conductors 121, 122, and 123 formed on the ferrite member 120 serve as inductors, and in order to match the impedance thereof to that of an external circuit, the capacitors 115 are additionally disposed in parallel. The resistor 114 is provided in addition to the central conductor 123 so as to permit the nonreciprocal circuit device 110 to act as an isolator allowing only the signals sent from the input/output terminal P1xe2x80x2 to the input/output terminal P2xe2x80x2 to pass.
Recently, with the demand for miniaturization of communication equipment, reduction in size of a nonreciprocal circuit device as one of the essential components incorporated therein has also been required.
In the lumped-constant nonreciprocal circuit device 110 described above, however, as shown in the equivalent circuit diagram of FIG. 11, each inductor and each capacitor constitutes a parallel-resonance circuit. Since the resonance frequency f of the circuit is substantially given by the formula f=1/{2xcfx80xc2x7(LC)xc2xd}, the higher the frequency of the nonreciprocal circuit device, the smaller the value of LC. As a result, the size of the nonreciprocal circuit device is reduced. For example, in the case of 2 GHz, the size of the nonreciprocal circuit device is approximately 7xc3x977 mm.
In this case, the higher the usable frequency, the smaller the nonreciprocal circuit device, with the result that the requirement for miniaturizing the device as a component used in communication equipment is satisfied. However, there is a problem in the manufacturing of the device. In other words, reduction in size of the nonreciprocal circuit device makes formation and connection of the central conductors complicated, leading to occurrences of variations in the manufacturing process among nonreciprocal circuit devices. Furthermore, the higher the frequency and the smaller the value of LC, the greater the influence of variations in manufacturing on characteristics of the nonreciprocal circuit devices. For instance, assuming that an error of 1 nH of inductance occurs in the manufacturing process, consider the degree of the influence on the nonreciprocal circuit device in the cases in which the initial inductances are 10 nH and 1 nH. That is, if the error in the manufacturing process is equal to 1 nH in both cases, when the initial inductance is 10 nH, the change ratio in the inductance is 10%, whereas when the initial inductance is 1 nH, the change ratio is 100%. Therefore, the smaller the initial inductance, the greater the influence on the resonance frequency, leading to occurrence of greater variations in the frequency characteristics of the nonreciprocal circuit device.
For such a reason, there is a limitation on the frequencies usable with a lumped-constant nonreciprocal circuit device. Consequently, from the manufacturing point of view, approximately 2 GHz is the maximum frequency usable with the lumped-constant nonreciprocal circuit device at present.
On the other hand, a nonreciprocal circuit device usable even in frequency bands above approximately 2 GHz is the distributed-constant nonreciprocal circuit device. As an example of this, a description will be given of a known conventional nonreciprocal circuit device referring to FIG. 12. FIG. 12 shows an exploded perspective view of the conventional nonreciprocal circuit device, which is ordinarily referred to as a xe2x80x9cY-shaped distributed-constant nonreciprocal circuit devicexe2x80x9d.
As shown in FIG. 12, a conventional nonreciprocal circuit device 140 comprises a ferrite member 120a, an electrode 150 formed on a surface thereof, a ground electrode formed on a back thereof, and an upper magnet and a lower magnet 142. The electrode 150 formed on the ferrite member 120a comprises a resonator 151 resonating in the TM110 mode at the center, and input/output connection electrodes 152, 153, and 154 formed in each of the three different directions from the resonator 151. Between the resonator 151 and the input/output connection electrodes 152, 153, and 154 are formed impedance converters 152a, 153a, and 154a having length of xcex/4 for the purpose of impedance matching. Additionally, the input/output connection electrodes 152, 153, and 154 are provided for being connected to an external circuit.
By applying a DC magnetic field with the upper and lower magnets 142, the nonreciprocal circuit device 140 functions as a circulator, in which signals from an input/output terminal P4xe2x80x2 pass through an input/output terminal P5xe2x80x2, signals from P5xe2x80x2 pass through an input/output terminal P6xe2x80x2, and signals from P6xe2x80x2 pass through P4xe2x80x2.
In the conventional nonreciprocal circuit device, the resonator formed on the surface of the ferrite member has a substantially circular shape. As a result, at the junction of the input/output connection electrode and the resonator, the electrode width is greatly increased to provide the impedance converter. Impedance matching would be impossible between the input/output connection electrode and the resonator, if they were connected directly without the impedance converter. Thus, in the conventional art, as shown in FIG. 12, in order to achieve impedance matching, the impedance converter must be connected to the input/output connection electrode near the resonator. Consequently, this leads to an increase in size of the nonreciprocal circuit device.
The above-described problems are solved by the present invention. The present invention provides a nonreciprocal circuit device capable of being manufactured for use in frequency bands of approximately 2 GHz or higher without adding an impedance converter or the like.
To this end, according to a first aspect of the present invention, there is provided a nonreciprocal circuit device including a magnetic body, a plurality of mutually intersecting central conductors disposed in proximity to the magnetic body, and a magnet for applying a DC magnetic field, in which the plurality of central conductors have lengths of substantially nxc2x7xcexg/2 (n=a natural number) with respect to a wavelength xcexg at a usable frequency. An end of each central conductor is an input/output terminal connected to an input/output, whereas the other end thereof is disconnected and therefore is an electrically open end.
In this arrangement, the length of each central conductor is set to be substantially nxc2x7xcexg/2 (n=a natural number) with respect to a wavelength xcexg at a usable frequency, and one of the two ends thereof is an open end. This permits the central conductor to serve as a xcexg/2-wavelength resonator. In other words, by the use of only the central conductor, a circuit equivalent to the parallel-resonance circuit having an inductor and a capacitor as shown in the equivalent circuit diagram of FIG. 11 can be formed. Moreover, by applying the DC magnetic field the nonreciprocal circuit device can be made, and there is no need to add a capacitor. Furthermore, since a narrow strip-line central conductor can be used in this device, no impedance converter need be connected in order to obtain impedance matching between the central conductors and the input/output connection electrodes. Additionally, since one end of each central conductor is an open end, there is no risk of a poor connection at that end. As a result, this contributes to reduction of factors degrading the reliability of a nonreciprocal circuit device.
According to a second aspect of the present invention, there is provided a nonreciprocal circuit device including a magnetic body, a plurality of mutually intersecting central conductors disposed in proximity to the magnetic body, and a magnet for applying a DC magnetic field, in which the plurality of central conductors have lengths of substantially (2mxe2x88x921)xc2x7xcexg/4 (m=a natural number) with respect to a wavelength xcexg at a usable frequency, and one of the ends of each central conductor is an input/output terminal connected to an input/output, and the other end thereof is a ground terminal connected to a ground.
The length of each central conductor is set to be substantially (2mxe2x88x921)xc2x7xcexg/4 (m=a natural number) with respect to a wavelength xcexg at a usable frequency, and one end of each central conductor is a ground terminal so as to permit each central conductor to serve as a xcexg/4-wavelength resonator. As a result, for a given frequency, this arrangement can make the length of the central conductor shorter than that in a case where the central conductor serves as a xcexg/2-wavelength resonator. Thus, further miniaturization of the nonreciprocal circuit device can be achieved.
In addition, in the nonreciprocal circuit device according to a third aspect of the present invention, the length of the central conductor may be substantially xcexg/2.
Furthermore, in the nonreciprocal circuit device according to a fourth aspect of the present invention, the length of the central conductor may be substantially xcexg/4.
These arrangements can minimize the size of the nonreciprocal circuit device since the length of the central conductor can be as short as possible.
Furthermore, in one of the nonreciprocal circuit devices described above, at least two of the central conductors have different widths. Since the central conductors mutually intersect with an insulation film therebetween, the respective positional relationships between the magnetic body and the central conductors differ. When there are such differences, effective dielectric constants are also different among the central conductors, with the result that the values of characteristic impedance thereof differ. If the central conductors have the same widths, variations in the characteristics of bandwidths or the like occur among the ports of the nonreciprocal circuit device. In contrast, if the widths of the central conductors are different, the bandwidth characteristics or the like among the ports can be adjusted in any desired way.
Furthermore, according to the present invention, there is provided a communications device including one of the nonreciprocal circuit devices described above, a transmission circuit, a receiving circuit, and an antenna. This permits a compact communications device to be produced.